7 research outputs found
Flavylium-Supported Poly(<i>N</i>āisopropylacrylamide): A Class of Multistimuli Responsive Polymer
A new
multistimuli responsive polymer was prepared by means of
the free-radical copolymerization of <i>N</i>-isopropylacrylamide
and a vinyl flavylium derivative. The polymer shows response to temperature
(conferred by the <i>N</i>-isopropylacrylamide units) and
pH and light stimuli (conferred by the flavylium moieties). The polymer
follows the typical pH dependent network of chemical reactions of
flavylium compounds in water that was studied by means of UVāvis
spectroscopy and stationary fluorescence emission. Regarding the temperature
response, the presence of the flavylium cation in the polymeric chains
reduces the LCST, and it is possible to take advantage of this effect
to photo-collapse partially the polymer by irradiating the system
in defined ranges of pH and temperature
Isomerization between 2-(2,4-Dihydroxystyryl)-1-benzopyrylium and 7-Hydroxy-2-(4-hydroxystyryl)-1-benzopyrylium
2-Phenyl-1-benzopyrylium (flavylium) and 2-styryl-1-benzopyrylium
(styrylflavylium) cations establish in aqueous solution a series of
equilibria defining chemical reaction networks responsive to several
stimuli (pH, light, redox potential). Control over the mole fraction
distribution of species by applying the appropiate stimuli defines
a horizontal approach to supramolecular chemistry, in agreement with
the customary bottom-up approach toward complex systems. In this work,
we designed an asymmetric styrylchalcone able to cyclize in two different
ways, producing two isomeric styrylflavylium cations whose chemical
reaction networks are thus interconnected. The chemical reaction networks
of 2-(2,4-dihydroxystyryl)-1-benzopyrylium (AH<sup>+</sup>) and 7-hydroxy-2-(4-hydroxystyryl)-1-benzopyrylium
(AH<sup>+</sup><sub>iso</sub>) comprise the usual species observed
in flavylium-derived networks, in this case, the styryl derivatives
of quinoidal bases, hemiketals, and chalcones. The thermodynamics
and kinetics of the crossed networks were characterized by the use
of UVāvis absorption and NMR spectroscopy as well as time-resolved
pH jumps followed by stopped-flow. The two styrylflavylium cations
are connected (isomerize) through two alternative intermediates, the
asymmetric <i>trans</i>-styrylchalcone (Ct) and a spiropyran-type
intermediate (SP). At pH = 1, AH<sup>+</sup> slowly evolves (<i>k</i><sub>obs</sub> ā 10<sup>ā5</sup> s<sup>ā1</sup>) to a mixture containing 62% AH<sup>+</sup><sub>iso</sub> through
the Ct intermediate, while at pH = 5, the SP intermediate is involved.
The observed rate constants for the conversion of the styrylflavylim
cations into equilibrium mixtures containing essentially Ct follow
a pH-dependent bell-shaped curve in both networks. While at pH = 1
in the dark, AH<sup>+</sup> evolves to an equilibrium mixture containing
predominantly AH<sup>+</sup><sub>iso</sub>, irradiation at Ī»
> 435 nm induces the opposite conversion
Design and Synthesis of Photoactive Ionic Liquids
Two
ionic liquids with photoisomerizable <i>p</i>-hydroxycinnamic
acid moieties were synthesized and characterized by X-ray crystallography
and DSC, and their photochemistry was studied in solution and neat
conditions. Irradiation at absorption maxima led to <i>transācis</i> photoisomerization and resulted in significant reduction of melting
temperatures of the ionic liquids. X-ray structures of both compounds
show an intricate network of supramolecular interactions before irradiation.
Physical and chemical transformations are completely reversible upon
irradiation at lower wavelengths of ionic liquid solutions in acetonitrile
Photorheological Ionic Liquids
Two
room temperature ionic liquids (ILs) bearing coumarin and <i>p-</i>hydroxycinnamic acid moieties are synthesized, and their
photochemistry is studied in solution and neat conditions. Irradiation
at absorption maxima leads to photochemical transformations and results
in changes of their rheological properties which are evaluated by
rotational rheometry. Samples of ionic liquids are also studied by
Hyper-Rayleigh scattering, and the effect of their photochemistry
on ionic nanoaggregation is discussed
Acenaphthylene-Based Chromophores for Dye-Sensitized Solar Cells: Synthesis, Spectroscopic Properties, and Theoretical Calculations
A set of acenaphthylene dyes with arylethynyl Ļ-bridges
was
tested for dye-sensitized solar cells (DSSCs). Crucial steps for the
extension of the conjugated system from the acenaphylene core involved
Sonogashira coupling reactions. Phenyl, thiophene, benzotriazole,
and thieno-[3,2-b]thiophene moieties were employed
to extend the conjugation of the Ļ-bridges. The systems were
characterized by cyclic voltammetry and by UVāvis absorption
and emission. The spectroscopic characterization showed that the last
three bridges resulted in red-shifted absorption and emission spectra
relative to the parent phenyl-bridged compound, in accordance with
TD-DFT calculations. The phenylethynyl derivative 6a achieved
a conversion efficiency of 2.51% with Voc, Jsc, and FF values of 0.365 V, 13.32
mA/cm2, and 0.52, respectively. The efficiency of this
compound improved to 3.15% with the addition of CDCA (10 mM), representing
the best efficiency result in this study. The overall conversion efficiency
of the other aryl derivatives 6bād proved to be significantly inferior (14ā40%) to that of 6a due to a significant decrease of Jsc
Impact of Water on the CisāTrans Photoisomerization of Hydroxychalcones
The
photochromism of a 2-hydroxychalcone has been studied in CH<sub>3</sub>CN and H<sub>2</sub>O/CH<sub>3</sub>OH (1/1, v/v), as well
as in analogous deuterated solvents using steady-state (UVāvis
absorption, <sup>1</sup>H and <sup>13</sup>C NMR) and time-resolved
(ultrafast transient absorption and nanosecond flow flash photolysis)
spectroscopies. Whereas the irradiation of <i>trans</i>-chalcone
(<b>Ct</b>) under neutral pH conditions leads to the formation
of the same final chromene derivative (<b>B</b>) in both media,
two distinct photochemical mechanisms are proposed in agreement with
thermodynamic and kinetic properties of the chemical reaction network
at the ground state. Following light excitation, the first steps are
identical in acetonitrile and aqueous solution: the FranckāCondon
excited state rapidly populates the <i>trans</i>-chalcone
singlet excited state <sup>1</sup><b>Ct</b>* (LE), which evolves
into a twisted state <sup>1</sup><b>P</b>*. This excited state
is directly responsible for the photochemistry in acetonitrile in
the nanosecond time scale (16 ns) leading to the formation of <i>cis</i>-chalcone (<b>Cc</b>) through a simple isomerization
process. The resulting <i>cis</i>-chalcone evolves into
the chromene <b>B</b> through a tautomerization process in the
ground state (Ļ = 10 ms). Unlike in acetonitrile, in H<sub>2</sub>O/CH<sub>3</sub>OH (1/1, v/v), the <b>P</b>* state becomes
unstable and evolves into a new state attributed to the tautomer <sup>1</sup><b>Q</b>*. This state directly evolves into <b>B</b> in one photochemical step through a consecutive ultrafast tautomerization
process followed by electrocyclization. This last case represents
a new hypothesis in the photochromism of 2-hydroxychalcone derivatives
2,2ā²-Spirobis[chromene] Derivatives Chemistry and Their Relation with the Multistate System of Anthocyanins
The chemistry of 2,2ā²-spirobisĀ[chromene]
derivatives is
intimately related to the one of anthocyanins and similar compounds.
The 2,2ā²-spirobisĀ[chromene] species plays a central role in
the network of chemical reactions connecting two different flavylium-based
multistate systems. In the present work, a new asymmetric 2,2ā²-spirobisĀ[chromene]
intermediate possessing a constrained propylenic bridge between carbons
3 and 3ā² was isolated and its role as a pivot in the anthocyanins-type
multistate of chemical reactions was investigated by the conjugation
of absorption spectroscopy, stopped-flow, NMR, and X-ray crystallography.
It was confirmed that the propylenic bridge is essential to stabilize
the spirobisĀ[chromene] species. Furthermore, under acidic conditions,
two <i>cis</i>ā<i>trans</i> styrylflavylium
isomers were identified, which could be interconverted directly into
one another with light. This is the first report of styrylflavylium
cations with photoisomerization on the styryl moiety